The present invention relates to the construction of a communication system for transmitting information by use of electrical or optical signals, and more particularly to a system construction in which a high-speed transmission signal apparatus is added in a transmission network with a new synchronous optical transmission apparatus or synchronous optical network (SONET) while using the existing low-speed transmission apparatus, thereby improving a transmission speed in the network and a method for structuring such a system construction.
A MUX (multiplex) device of the new synchronous transmission apparatus (hereinafter referred to as SONET transmission apparatus) multiplexes signals from a plurality of low-speed side interface devices, for example, signals from four 155.52 Mbps interface devices to a 622.08 Mbps signal in the MUX device and delivers the multiplexed signal from a high-speed side interface device. On the other hand, when receiving a 622.08 Mbps signal, the MUX device can separate the received signal into, for example, four 155.52 Mbps signals (in the case where four 155.52 Mbps signals are included in the received signal) so that the separate signals are distributed to the 155.52 Mbps interface devices, respectively.
In the case of a SONET apparatus having an add drop MUX device (hereinafter referred to as ADM), there are provided two systems of high-speed side interface devices one of which is called "east" and the other of which is called "west". Each of the east and west high-speed side interfaces uses a system which is provided with one system of transmitting/receiving function or a system which is provided with two systems of transmitting/receiving functions. The former system is called "two-fiber" and the latter system is called "four-fiber".
In the case where a 622.08 Mbps signal received from one of the two high-speed side interface devices includes, for example, four 155.52 Mbps signals, the ADM device can deliver a part of the four 155.52 Mbps signals directly to the other high-speed side interface device while transmitting the remaining 155.52 Mbps signals to low-speed side interface devices. Also, the ADM device can transmit a signal (of, for example, 155.52 Mbps) received from the low-speed side interface device to the high-speed side interface device so that it is multiplexed to a part of a 622.08 Mbps signal.
In a system construction using the above-mentioned ADM device, a signal which is delivered from one of the high-speed side interface devices HSIF directly to the other high-speed side interface device HSIF is called "THROUGH", as exemplified in FIG. 1. A signal which is delivered from the high-speed side interface device HSIF to the low-speed side interface device LSIF is called "DROP". A signal which is delivered from the low-speed side signal device LSIF to the high-speed side signal device HSIF, is called "ADD". A signal received from the low-speed side interface device LSIF or the high-speed side interface device HSIF which is transferred to two or more interface devices at a time is called "BROADCAST".
A signal inputted to the high-speed side interface device HSIF which is outputted from the same high-speed side interface device HSIF which again through no switch SW, is called "LOOP BACK".
A signal inputted to the low-speed side interface device LSIF which is outputted from the same low-speed side interface device LSIF again through no switch SW, is called "HAIR PIN" connection.
In the above-mentioned example, it is possible to transmit and receive a signal of at most 622.08 Mbps as the high-speed signal in the case where the ADM or the MUX device has already been installed.
In the case where an ADM or MUX device as mentioned above has already been installed, there will be generated a need to install low-speed side interface devices additionally. In the case where the additional installation of low-speed side interfaces results in the total capacity of low-speed side interfaces exceeding the transmission capacity of high-speed side interfaces, a high-speed side interface device having a larger capacity will be required. The standard of the speed of a high-speed side interface device possessed by a SONET apparatus is prescribed as 51.84 Mbps, 155.52 Mbps, 622.08 Mbps, 1244.16 Mbps and 2488.32 Mbps. In general, devices having the 51.84 Mbps, 155.52 Mbps, 622.08 Mbps, 1244.16 Mbps and 2488.32 Mbps interface devices on the high-speed sides thereof are called OC-1ADM, OC-3ADM, OC-12ADM, OC-24ADM and OC-48ADM devices, respectively.
In this application, systems having 51.84 Mbps, 155.52 Mbps, 622.08 Mbps, 1244.16 Mbps and 2488.32 Mbps interfaces on the high-speed sides thereof are called OC-1, OC-3, OC-12, OC-24 and OC-48 systems, respectively.
In the prior art, in the case where the additional installation of low-speed side interface devices is required and the transmission capacity of the system is insufficient as a result, there is required, for example, an OC-48 system having a 2488.32 Mbps interface device, as a high-speed interface which has a capacity larger next to 622.08 Mbps. The increase of capacity from the OC-12 system to the OC-48 system was realized in such a manner that when an OC-48ADM device provided with a high-speed side interface device having a capacity larger than 622.08 Mbps is placed (or installed), the high-speed side interfaces of at most four existing OC-12ADM devices are connected to low-speed side interface devices of the OC-48ADM device, and the connection of a transmission path connected to the high-speed side interface device of the OC-12ADM device is changed to the high-speed side interface device of the OC-48ADM device. Therefore, the maximum construction of the whole system requires four OC-12ADM devices and one OC-48ADM device, which is disadvantageous not only in the economical aspect but also in the aspect of installation space.